1. <Field of the Invention>
The present invention relates to a positive-electrode material, particularly a positive-electrode material including lithium vanadium phosphate, a lithium-ion secondary battery using the positive-electrode material, and method of manufacturing the positive-electrode material.
2. <Related Art>
Recently, electric storage devices such as a lithium-ion secondary battery are used as a power source of electric apparatuses, and also a power source of electric vehicles (electric vehicle (EV), hybrid electric vehicles (HEV), etc.). The electric storage devices such as the lithium-ion secondary battery are required to have more improved properties, for example, improved energy density (high capacity), improved power density (high output power) or a cycle characteristic (improved cycle life span), high safety.
In many of lithium-ion secondary batteries used in small electric apparatuses, LiCoO2 or LiCo1-x-yM1xM2yO2 (M1, M2=metallic elements) with a crystal structure similar to LiCoO2 is used as an active material so as to achieve an electric storage device with high capacity and long life span. However, these positive-electrode active materials have problems that, at a high temperature and a high electric potential when an abnormality occurs, the positive-electrode active materials excessively react with an electrolyte to generate a heat while releasing oxygen. In the worst case, they may ignite.
In recent years, as positive-electrode active materials which have superior heat stability even at a high temperature and a high electric potential, poly-anion-based positive-electrode materials, for example, an olivine-type Fe(LiFePO4) or an olivine-type Mn(LiMnPO4) with a crystal structure similar to the olivine-type Fe(LiFePO4), are considered. Such positive-electrode active materials have practically been used, for example, in electrical tools. These poly-anion-based materials have a strong crystal structure and do not easily release oxygen even at a high temperature and a high voltage.
However, an operating voltage of LiFePO4 with respect to Li/Li+ reference is 3.3 to 3.4 V and is lower than an operating voltage of a positive-electrode material used in general batteries. Thus, LiFePO4 is insufficient in an aspect of energy density or power density. An operating voltage of LiMnPO4 with respect to Li/Li+ reference is 4.1 V, and LiMnPO4 has theoretical capacity of 160 mAh/g. Thus, as for LiMnPO4, a battery with high energy density may be expected. However, a resistance of a material itself of the LiMnPO4 is high, and it is difficult to increase energy density.
Accordingly, development of a positive-electrode material with a strong crystal structure and high operating voltage has been required.
In recent years, NASICON-type lithium vanadium phosphate namely, Li3V2(PO4)3 attracts attention (for example, Patent Document 1: JP-A-2001-500665), as a positive-electrode material having excellent heat stability due to a strong crystal structure, and high operating voltage. The Li3V2(PO4)3 has high electric potential of 3.8 V(compared to Li+/Li electric potential), and high energy density of 120 to 130 mAh/g when charging 4.2 V and 150 to 180 mAh/g when charging 4.6V.
However, Li3V2(PO4)3 described in the patent reference 1 has a tendency to be insufficient in a rate characteristic of the lithium-ion secondary battery because of having low electric conductivity compared to LiCoO2 and the likes.
Accordingly, when Li3V2(PO4)3 is used as an electrode active material, it is necessary to coat a plenty of conductive aids on a face of the material.
However, by the plenty of the conductive aids, an existing rate of an active material in an electrode decreases, and an energy density is decreased. Moreover, when Li3V2(PO4)3 is repetitively used, a cycle characteristic is decreased.